Ever wondered how to measure if a product is truly user-friendly? Enter the System Usability Scale (SUS)—a simple, reliable tool that reveals the real usability of any system.
What Is the System Usability Scale (SUS)?
The System Usability Scale, commonly known as SUS, is a 10-item questionnaire designed to assess the perceived usability of a system, product, or service. Developed in the late 1980s by John Brooke at Digital Equipment Corporation, SUS has become one of the most widely adopted usability measurement tools across industries—from software and websites to medical devices and mobile apps.
Origins and Development of SUS
The SUS was created during a time when usability testing was becoming increasingly important in human-computer interaction (HCI). John Brooke needed a quick, reliable method to evaluate system usability without requiring extensive resources or time. The result was a lightweight, yet powerful, ten-question survey that could be administered easily and produce consistent, quantifiable results.
Despite its age, SUS remains remarkably relevant. Its longevity is a testament to its simplicity and robustness. Unlike more complex usability frameworks, SUS doesn’t require specialized training to administer or interpret, making it accessible to designers, developers, product managers, and UX researchers alike.
One of the key strengths of the SUS is its agnostic nature—it can be applied to virtually any interactive system, regardless of platform or domain. Whether you’re testing a banking app, a smart home device, or a government portal, SUS provides a standardized way to measure user experience.
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Structure of the SUS Questionnaire
The SUS consists of ten statements, each rated on a five-point Likert scale ranging from “Strongly Disagree” (1) to “Strongly Agree” (5). The odd-numbered items are positively worded, while the even-numbered items are negatively worded, helping to reduce response bias.
Here are the ten standard SUS questions:
I think that I would like to use this system frequently.I found the system unnecessarily complex.I thought the system was easy to use.I think that I would need the support of a technical person to be able to use this system.I found the various functions in this system were well integrated.I thought there was too much inconsistency in this system.I would imagine that most people would learn to use this system very quickly.I found the system very cumbersome to use.I felt very confident using the system.I needed to learn a lot of things before I could get going with this system.Notice how the phrasing alternates between positive and negative..
This design helps balance user responses and minimizes the impact of acquiescence bias—where users tend to agree with statements regardless of content..
Scoring and Interpretation of SUS Results
Scoring the SUS is straightforward but requires careful attention to the alternating positive and negative items. For each odd-numbered item (positively worded), you subtract 1 from the user’s response (e.g., a response of 4 becomes 3). For even-numbered items (negatively worded), you subtract the user’s response from 5 (e.g., a response of 4 becomes 1).
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After transforming the scores, you sum them up and multiply the total by 2.5 to get a final score between 0 and 100. This scaling ensures that the average SUS score across thousands of studies—typically around 68—falls comfortably in the middle of the range.
Interpreting the score is equally important. While there’s no universal “pass/fail” threshold, general guidelines suggest:
- Below 50: Poor usability
- 50–65: Below average
- 65–75: Acceptable
- 75–85: Good
- 85+: Excellent
These benchmarks are based on extensive research and meta-analyses. For example, a study by Sauro and Lewis (2006) analyzed over 200 SUS scores and established percentile rankings that help contextualize results. A score of 80, for instance, places a system in the 70th percentile—better than 70% of all systems tested.
“The System Usability Scale is not just a score—it’s a conversation starter about user experience.” — Jeff Sauro, MeasuringU
Why the System Usability Scale Matters in UX Design
In the fast-paced world of digital product development, usability can’t be an afterthought. The System Usability Scale provides a fast, reliable way to quantify user experience, making it an essential tool in the UX designer’s toolkit. Unlike observational methods or task success rates, SUS captures subjective user perceptions—what users feel about a system, not just what they do.
Quantifying Subjective User Experience
Usability isn’t just about efficiency or error rates; it’s also about user satisfaction. A system might allow users to complete tasks quickly, but if they find it frustrating or confusing, long-term adoption will suffer. SUS bridges the gap between objective performance metrics and subjective user sentiment.
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For example, two e-commerce platforms might have identical task completion times, but one could score significantly higher on SUS. This difference often reflects better information architecture, clearer navigation, or more intuitive design—factors that aren’t always captured in task-based metrics.
By quantifying subjective experience, SUS enables teams to track improvements over time, compare design iterations, and justify UX investments to stakeholders. A rising SUS score after a redesign is powerful evidence that changes are positively impacting user perception.
Benchmarking and Competitive Analysis
One of the most powerful uses of SUS is benchmarking. Because it’s standardized, SUS scores can be compared across different products, versions, or even industries. This allows companies to answer critical questions: Is our new app more usable than the old one? How does our product stack up against competitors?
For instance, a fintech startup might conduct a usability test on its mobile banking app and get a SUS score of 72. By testing a leading competitor’s app under similar conditions, they might find a score of 80. This 8-point gap highlights a usability deficit and provides a clear target for improvement.
Organizations like MeasuringU have compiled extensive databases of SUS scores across domains. According to their research, the average SUS for mobile apps is around 70, while desktop software averages 74. Knowing these benchmarks helps teams set realistic goals and interpret their results in context.
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Moreover, SUS can be used longitudinally. By measuring SUS at multiple points in a product’s lifecycle—after launch, after major updates, after user onboarding changes—teams can build a usability trend line that informs strategic decisions.
Supporting Data-Driven Design Decisions
In modern product development, decisions are increasingly driven by data. SUS provides a clear, numerical metric that can be integrated into dashboards, sprint retrospectives, and executive reports. When a design team proposes a new interface, they can back it up with SUS data from prototypes or A/B tests.
For example, a team might test two versions of a checkout flow. Version A scores 68 on SUS, while Version B scores 82. Even if both versions have similar conversion rates, the higher SUS score suggests that Version B offers a better user experience, potentially leading to higher customer satisfaction and retention.
Additionally, SUS scores can be correlated with other business metrics. Research has shown that higher SUS scores are linked to increased customer loyalty, reduced support costs, and higher conversion rates. A study by the Nielsen Norman Group found that usability improvements leading to higher SUS scores often result in measurable ROI, especially in e-commerce and SaaS environments.
How to Administer the System Usability Scale Effectively
While the SUS is simple to use, administering it effectively requires attention to timing, context, and methodology. Poorly timed surveys or inappropriate participant selection can skew results and lead to misleading conclusions.
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Best Practices for Survey Timing and Context
The ideal time to administer the SUS is immediately after a usability test or user interaction session. This ensures that the user’s experience is fresh in their mind, reducing recall bias. Delaying the survey by hours or days can lead to inaccurate responses, as users may forget specific pain points or positive moments.
It’s also important to consider the context in which the SUS is given. For example, if users are frustrated during testing due to technical issues (e.g., slow loading times, bugs), their SUS scores may reflect those frustrations rather than the core usability of the interface. Researchers should document such anomalies and, if necessary, exclude affected responses from analysis.
Additionally, SUS should be used as a post-task or post-session questionnaire, not as a standalone survey without any interaction. The scale is designed to measure usability after use, not before. Asking users to rate a system they haven’t interacted with defeats the purpose and invalidates the results.
Selecting the Right Participants
Participant selection is critical for valid SUS results. Ideally, participants should represent the actual or target user base. Testing a medical device with software engineers, for example, may yield high SUS scores—but those results won’t reflect how real healthcare professionals experience the system.
While SUS can be used with as few as 5–8 users in qualitative studies, larger sample sizes (20+) are recommended for reliable benchmarking or comparative analysis. Small samples can be misleading due to outliers. For instance, one extremely negative user can drag an average score down significantly.
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It’s also important to consider user diversity. Including participants with varying levels of technical proficiency, age, and familiarity with similar systems ensures that the SUS score reflects a broader user experience. This is especially important for public-facing services like government portals or educational platforms.
Combining SUS with Other Usability Methods
SUS is most powerful when used alongside other usability evaluation methods. While it provides a high-level usability score, it doesn’t explain why users feel a certain way. Combining SUS with qualitative methods like think-aloud protocols, interviews, or observational notes can provide deeper insights.
For example, a SUS score of 55 might indicate poor usability, but follow-up interviews could reveal that users struggled with unclear error messages or confusing navigation labels. This combination of quantitative and qualitative data enables more targeted design improvements.
Other complementary tools include the Net Promoter Score (NPS) for loyalty, the UMUX (Usability Metric for User Experience) for a shorter alternative, or eye-tracking for behavioral analysis. Using SUS in a mixed-methods approach creates a more complete picture of the user experience.
Advantages of the System Usability Scale
The enduring popularity of the System Usability Scale isn’t accidental. Its widespread adoption is due to a combination of practical advantages that make it ideal for both academic research and real-world product development.
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Simplicity and Ease of Use
One of the biggest strengths of SUS is its simplicity. The questionnaire takes less than 5 minutes to complete, making it non-intrusive and user-friendly. Unlike lengthy surveys that can fatigue participants, SUS respects users’ time while still delivering valuable data.
Its straightforward scoring system also makes it accessible to non-researchers. Product managers, developers, and designers can calculate and interpret SUS scores without needing a background in statistics or psychology. This democratization of usability measurement empowers cross-functional teams to engage with user experience data.
Reliability and Validity
Despite its brevity, SUS is remarkably reliable. Numerous studies have confirmed its internal consistency (Cronbach’s alpha typically above 0.9), meaning the ten items are measuring the same underlying construct—usability.
It also demonstrates strong validity, correlating well with other usability metrics like task success rates, time-on-task, and user satisfaction scales. A meta-analysis by Bangor, Kortum, and Miller (2008) confirmed that SUS is a valid and reliable tool across a wide range of applications and user populations.
This reliability makes SUS suitable for both formative (early-stage) and summative (final evaluation) testing. Whether you’re iterating on a prototype or evaluating a final product, SUS delivers consistent, trustworthy results.
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Wide Applicability Across Domains
From healthcare to aviation, education to entertainment, SUS has been successfully applied in countless domains. Its language is neutral and generic enough to work with digital and physical systems alike.
For example, SUS has been used to evaluate:
- Electronic health records (EHR) systems in hospitals
- In-vehicle infotainment systems
- Online learning platforms
- Smartphone apps for seniors
- Industrial control panels
This versatility is rare among usability tools. Most are tailored to specific contexts, but SUS’s broad applicability makes it a go-to choice for organizations with diverse product portfolios.
“The beauty of SUS is that it works everywhere—no customization needed.” — Dr. Aaron Bangor, Human Factors Researcher
Limitations and Criticisms of the System Usability Scale
While the System Usability Scale is widely respected, it’s not without limitations. Understanding these weaknesses is crucial for using SUS effectively and avoiding misinterpretation of results.
Lack of Diagnostic Detail
SUS provides a single overall score, but it doesn’t tell you why a system scored poorly. A low score could be due to navigation issues, poor feedback, confusing terminology, or slow performance—but SUS alone won’t pinpoint the cause.
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This is why SUS should never be used in isolation. Without follow-up questions or observational data, teams may waste time fixing the wrong problems. For example, redesigning a menu structure based on a low SUS score might not help if the real issue is slow loading times.
To address this, some researchers supplement SUS with open-ended questions like: “What did you find most difficult about the system?” or “What one change would improve your experience the most?” These qualitative insights add depth to the quantitative score.
Sensitivity to Context and Framing
SUS scores can be influenced by factors outside the system’s design. For instance, users who are tired, stressed, or unfamiliar with testing environments may give lower ratings, even if the system is well-designed.
Similarly, the way the survey is introduced can affect responses. If a facilitator says, “We’ve spent months improving this system,” users may feel pressured to give higher scores. Conversely, if they’re told, “We know this version has issues,” they may be more critical.
To minimize framing effects, it’s best to present the SUS neutrally: “We’d like your feedback on your experience using the system.” Avoid leading language or emotional cues.
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Not Designed for All User Groups
While SUS works well for literate, tech-savvy adults, it may not be suitable for children, elderly users with cognitive decline, or individuals with low literacy. The abstract nature of the statements (e.g., “I found the system unnecessarily complex”) can be difficult to interpret for some populations.
In such cases, alternative tools like the SUPR-Q (for websites) or the QUIS (Questionnaire for User Interaction Satisfaction) may be more appropriate. Alternatively, SUS can be adapted with simpler language or administered verbally with assistance.
Alternatives and Complements to the System Usability Scale
While SUS is a gold standard, it’s not the only usability measurement tool available. Depending on your goals, timeline, and audience, other scales may offer advantages—or work better in combination with SUS.
UMUX and UMUX-Lite
The Usability Metric for User Experience (UMUX) is a four-item questionnaire based on ISO standards for usability. It’s shorter than SUS and focuses on two core dimensions: usability and efficiency.
UMUX-Lite, a two-item version, is even more concise and correlates strongly with SUS (r = 0.87). It’s ideal for situations where survey length is a concern, such as in-app micro-surveys.
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However, UMUX lacks the robust benchmarking data that SUS has accumulated over decades. While it’s a valid alternative, SUS still holds an edge in terms of historical context and interpretability.
Learn more about UMUX at MeasuringU’s UMUX guide.
Nielsen Norman Group’s SUS Variants
The Nielsen Norman Group has developed several SUS-inspired tools tailored to specific contexts. For example, the SUPR-Q (Standardized User Experience Percentile Rank Questionnaire) measures website usability, trust, appearance, and loyalty.
SUPR-Q includes SUS-like items but adds dimensions relevant to web experiences. It’s particularly useful for e-commerce, media, and service websites where trust and aesthetics play a big role in user satisfaction.
Another variant is the PSSUQ (Post-Study System Usability Questionnaire), which is longer and more detailed, making it suitable for in-depth usability studies in controlled environments.
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Custom Usability Questionnaires
Some organizations develop their own usability scales tailored to their products or industries. While this allows for greater specificity, it comes at the cost of comparability. Custom scales can’t be benchmarked against industry averages, and their reliability must be validated independently.
For most teams, starting with SUS and adding 1–2 custom questions is a better approach. This preserves the benefits of standardization while capturing domain-specific insights.
Real-World Applications of the System Usability Scale
The true value of SUS lies in its real-world impact. Across industries, organizations use SUS to improve products, validate design decisions, and enhance user satisfaction.
Case Study: Healthcare Software Redesign
A major hospital system was struggling with low adoption of its new electronic health record (EHR) platform. Clinicians complained about complexity and inefficiency. A UX team conducted usability tests with 15 doctors and nurses, administering SUS before and after a redesign.
Initial SUS score: 52 (Poor)
Post-redesign SUS score: 78 (Good)
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The 26-point improvement was accompanied by qualitative feedback highlighting better workflow integration and reduced clicks. The SUS data helped secure executive buy-in for further UX investments and was used in training materials to demonstrate progress.
Case Study: Mobile Banking App Optimization
A fintech company wanted to improve its mobile app’s user experience. They tested three design variants with 30 users each, measuring SUS after a series of banking tasks.
Variant A: 65
Variant B: 71
Variant C: 83
Based on the SUS results, Variant C was selected for rollout. Post-launch analytics showed a 15% increase in feature usage and a 20% drop in customer support calls related to navigation—validating the SUS findings.
Academic and Government Use
SUS is widely used in academic research to evaluate new interaction techniques, assistive technologies, and educational software. Its standardized nature makes it ideal for publishing comparative studies.
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Government agencies also use SUS to assess public-facing digital services. For example, the U.S. Digital Service uses SUS to evaluate federal websites, ensuring they meet usability standards for all citizens, including those with limited digital literacy.
Explore real SUS case studies at Usability.gov, a resource by the U.S. Department of Health and Human Services.
What is a good System Usability Scale score?
A score of 68 is considered average. Anything above 75 is good, and above 85 is excellent. However, context matters—compare your score to industry benchmarks and previous versions of your product.
Can I modify the SUS questionnaire?
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While you can rephrase items for clarity, it’s not recommended to change the core questions, as this affects comparability. If you must modify it, clearly label it as a “SUS-inspired” scale, not SUS.
How many users do I need for a reliable SUS score?
For quick feedback, 5–8 users can suffice. For benchmarking or publication, aim for 20+ users to ensure statistical reliability.
Is the System Usability Scale free to use?
Yes, SUS is in the public domain and free for both commercial and academic use. No permission is required, though proper citation is appreciated.
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Can SUS be used for non-digital systems?
Absolutely. SUS has been used to evaluate physical products like medical devices, kiosks, and even household appliances. The key is that the system must be interactive.
The System Usability Scale remains one of the most effective, efficient, and versatile tools for measuring user experience. Its simplicity, reliability, and broad applicability make it indispensable for anyone involved in design, development, or product management. While it has limitations, especially in diagnostic depth, its value is amplified when used alongside qualitative methods and other metrics. By understanding how to administer, interpret, and act on SUS data, organizations can make smarter, user-centered decisions that lead to better products and happier users.
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